This invention relates to junction termination structures in high voltage power semiconductor devices and more particularly to junction termination structures for wide bandgap power devices, such as silicon carbide, gallium nitride, gallium aluminum nitride, gallium indium nitride, diamond devices, and the like.
Junction termination structures are a common feature of most high voltage power switching devices, such as p-n diodes or Schottky diodes, and bipolar transistors, thyristors, power MOSFETs and IGBTs, which intrinsically include one or more p-n diodes. The intent of these structures is to reduce high electric fields at the edge of the device, resulting in higher blocking voltages.
A standard approach is to add a junction termination structure that reduces the high field at the edges by providing additional charges on which the field lines terminate. This charge is typically obtained by implanting dopants of appropriate polarity in a concentric region around the junction area, which fully depletes as a high voltage is applied across the diode. Up to three such implants are typically used, with different doses to further reduce the field spikes at the transition between each region. Alternatively, one can implant a set of concentric rings, called guard rings, that provide a similar function. Yet another approach is to etch a doped region to a thickness that provides the desired dose. Both step-like concentric regions and concentric rings can be etched this way as an alternate approach to the ion implanted structures.
Termination structures have been used for a variety of devices such as p-n diodes, Schottky diodes and bipolar transistors. A typical vertical structure of these devices and the location of the termination structure are illustrated with FIGS. 1A, 1B and 1C. Note that in these constructions, the unetched portions of the termination structure has a dopant polarity opposite to that of the substrate.
Prior art includes both ion implanted as well as etched structure with concentric regions with variable thickness or dose and concentric rings. An example of a three section etched termination structure is shown in FIGS. 2A and 2B, and an example of an etched termination with concentric rings is shown in FIGS. 2C and 2D. Unlike the structures shown schematically in the figures, the layered structures in FIGS. 2A and 2B can have variable etch depth and spacing and concentric ring structure in FIGS. 2C and 2D can have variable width in the unetched portions as well as in the spacing.
All these approaches would benefit from a larger number of regions with different dose or, better yet, a controlled taper, which is quite complex and more difficult to control.